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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

404

LINSEED  OIL

chemical and physical characters, the following special tests may be made
on this oil:

1.  Reaction   of  the   Hexabromo-compounds.—When   vigorously
shaken in a stoppered cylinder with 100 c.c. of a mixture of 28 vols. of glacial
acetic acid, 4 vols. of nitrobenzene and I vol. of bromine (Halphen's bromine
reagent], 5 c.c. of the oil give a yellow precipitate composed of hexabromo-
compounds of linolenic acid.   This precipitate is soluble in boiling benzene
and melts undecomposed at 175-180° (difference from oils of marine animals ;
see Fish Oils).

Drying walnut and hempseed oils behave like linseed oil, but other
vegetable and animal oils (excepting those of marine animals) give no
precipitate or only a slight turbidity with the bromine reagent.

2.  Drying Properties.—Linseed oil, being a drying oil, readily absorbs
oxygen, its drying power increasing with the rapidity with which it absorbs
its maximum proportion of oxygen.    Under the conditions of Livache's
method (see General Methods, 22, Drying Properties of Oils), a good linseed
oil absorbs about 14% of oxygen in two days, while by Bishop's method,
the maximum is 17% and is reached in 24 hours.

3.  Detection  of Adulterations.—Linseed  oil  may   be   adulterated
with other vegetable oils (especially colza, cottonseed, sesame, poppyseed,
cameline, hempseed and other drying oils) or with animal, mineral or resin
oils, which are tested for as follows :

i. OTHER VEGETABLE OILS. Marked addition of other vegetable oils
(especially if non-drying) to linseed oil generally lowers the iodine number
and the Maumene number and a linseed oil with an iodine number below
165 and a Maumene number less than 120° is to be suspected. In par-
ticular, colza oil may be detected by the Tortelli and Fortini test, cotton-
seed oil by the Halphen and the silver nitrate reactions, and sesame oil
by the furfural reaction (see these oils). The presence of drying oils (poppy-
seed, cameline and the like) is moderately difficult to ascertain, since these
oils do not possess special colour reactions.

. 2. FISH AND OTHER MARINE ANIMAL OILS. The presence of these oils
may be detected by the Halphen and Marcusson octabromo-compound
test and the Tortelli and Jaffe colour reaction (see Fish Oils).

3.  MINERAL OILS.   These are detectable by the fact that they lower
the density, the iodine number and the saponification number of linseed oil ;

: they may, in addition, be tested for in the unsaponifiable part (see General
Methods, 19, Unsaponifiable Substances),

4.  RESIN OILS.   These may be recognised by the odour, by the colour
reaction with sulphuric acid (see Resin Oils) and by the rotatory power
(linseed oil is almost inactive, whereas resin oils are decidedly dextro-
rotatory) ;   further, they increase the density and lower the iodine and
saponification numbers.

*
* *

Free acids (calculated as oleic acid) up to 7% are allowable in linseed oil,
unsapomfiable substances up to 1-5%, and moisture and various other impurities
up .to- i/o. For the genuine oil, D =0-930-0-934, iodine number = 164-191,
Zeiss refractometnc degree at 25° = 87-5, solidification point of the fatty acids

"~     °~'nd chemical characters are given in